Modern electronics systems are increasingly making use of low supply voltages. For example, digital processors at one time used voltages of ten or more volts, but the supply voltages have decreased over the years, and are now often in the 3-volt region. Nevertheless, the power consumption has remained substantially constant. Thus, direct supply voltages have tended to decrease, and currents have tended to increase. Transformers which produce such low voltages from alternating-current lines tend to become less efficient as the transformation ratio increases. One of the problems associated with transformer design is to maintain high efficiency at lower direct supply voltages, but at the same power.
The requirements of modern equipment tend to favor smaller and lighter-weight designs. Computers, for example, require low voltages and high currents, and the desire for portability of computers creates a powerful incentive for small and lightweight power supplies. Concomitant and even more severe requirements are placed on power supplies for use on spacecraft. Higher switching frequencies than the normal 60 Hz power-line frequency have been used over the years in order to achieve smaller volume and overall dimensions in switching converters. For example, present-day switching power supplies often use switching frequencies greater than 0.5 MHz.
Power transformers have been made using disk-like winding structures, as detailed in an article entitled "A COMPARATIVE STUDY OF LOW-PROFILE POWER MAGNETICS FOR HIGH-FREQUENCY, HIGH-DENSITY SWITCHING CONVERTERS", by Ramakrishnan et al., published at pp 388-394 of Volume 1 of APEC '97, the proceedings of the Annual Applied Power Electronics Conference and Exposition, sponsored by the IEEE, Feb. 23-27, 1997. The disk-type structures are made up of a plurality of dielectric layers, which are stacked vertically. Each of the dielectric layers has a central aperture which fits over the magnetic core. Each layer of the dielectric carries a pattern of conductor windings which loops around the central aperture, so as to define one or more windings about the core when the structure is assembled. The constraints of available materials and fabrication techniques results in a profile having a height of greater than 0.15 inch for viable magnetic designs. Some requirements are for profiles of less than 0.1 inch to satisfy packaging requirements.
Tube-type windings are also described in the abovementioned Ramakrishnan article. The tube-type windings therein described include a magnetically permeable core with an E-section and an I-section, together defining a single pole or center post. The primary and secondary windings are in the form of a flat tube dimensioned to fit over the center post of the E-section of the core. This type of winding is reported to produce 50 watts in a structure no larger than a quarter-dollar coin.
Improved planar transformer structures are desired.